An isolated buffer perfused guinea-pig heart preparation has been used to study the antiarrhythmic and cellular electrophysiological effects of alpha adrenoceptor blockade during myocardial ischaemia and reperfusion. Phentolamine 5 X 10(-6) M and indoramin 2 X 10(-6) M significantly reduced the incidence of ventricular tachycardia (VT) and ventricular fibrillation (VF) during ischaemia and reperfusion. Both drugs prolonged action potential duration (APD) and refractory period and reduced Vmax during normal perfusion and these effects were maintained during ischaemia. Reperfusion led to prompt recovery in all hearts but with initial transient shortening of APD. Phentolamine and indoramin abolished shortening of the APD whether added prior to ischaemia or immediately prior to reperfusion, and attenuated the reduction in refractory period, due to higher values at the end of ischaemia. Myocardial catecholamine depletion also significantly reduced ventricular tachycardia and ventricular fibrillation during ischaemia and reperfusion. Catecholamine depletion had similar effects on APD, refractory period and conduction, but phentolamine produced no additional effects when added to catecholamine depleted hearts suggesting that the effects observed here are mediated via adrenergic rather than direct myocardial effects.